1. Field of the Invention
This invention relates to a multilayer varistor of the structure wherein ceramic layers having varistor characteristics are stacked between internal electrode layers.
2. Prior Art
Varistors are utilized in low-voltage electronic equipment for absorbing high-voltage pulsative noise while bypass capacitors are utilized for removing low-voltage and high-frequency alternating noise. An advanced device having both the functions is a ceramic capacitor with varistor performance. A multilayer structure was proposed for the ceramic capacitor for reducing its size without detracting from its capacitance and reliability as disclosed in Japanese Patent Application Kokai (JP-A) Nos. 82386/1993, 275270/1993, 176954/1994, and 340472/1994.
JP-A 82386/1993, 275270/1993, and 176954/1994 disclose grain boundary insulated semiconductor ceramics based on SrTiO.sub.3, which cannot fully remove alternating noise and are less effective in suppressing the rise of pulsative noise since their capacitance is low.
In JP-A 340472/1994, a multilayer ceramic capacitor with varistor performance is fabricated using a ceramic composition comprising a dielectric component containing 100 mol of BaTiO.sub.3, 0.01 to 2 mol of MnO, and 0.01 to 10 mol of Y.sub.2 O.sub.3, 0.01 to 1 mol of Cr.sub.2 O.sub.3, and 0.5 to 10 mol of at least one glass component selected from BaSiO.sub.3, CaSiO.sub.3, and (Ba.sub.x Ca.sub.1-x)SiO.sub.3 wherein 0.43.ltoreq.x.ltoreq.0.62. Chromium oxide (Cr.sub.2 O.sub.3) is added to the ceramic composition in order to provide varistor characteristics. This multilayer ceramic capacitor has a high dielectric constant and a high capacitance, but its surge resistance is insufficient in some applications due to the presence of chromium. In automotive electric circuits, for example, surge voltage often occurs from the motor, spark plugs and static electricity and can cause short-circuit failure of the capacitor.